The Logribmet study is a prospective study that evaluate grip strength changes in both hands of postmenopausal women with breast cancer receiving treatment with tamoxifen or an aromatase inhibitor. Patients included in the study will undergo a grip strength measurement and receive a rheumatological questionnaire prior to the start of treatment, then at 3 and 6 months and at year 1 into therapy. Furthermore, BMI will be measured to assess if BMI is a predictor for the decrease in grip strength.